Applies to:

Symptoms

RMAN restore from sbt backup is very slow compared to backup speed.
Backup rate is 60-90Mb/sec.
Restore rate is 6Mb/sec.
Backup is hardware multiplexed but media manager does support multiplexed backups.
Restore validate runtime is VERY fast so problem is the write back to disk.

Cause

RMAN is using a 128Kb buffer size for the datafiles when restoring back to disk.
The input buffer size used during backup is 1Mb.
To confirm that you are hitting the same problem, query v$backup_async IO and check buffer_size for the backup eg

Solution

Change the buffer size by setting the following hidden parameter:
SQL>alter system set "_BACKUP_KSFQ_BUFSZ"=1048576;
Users restoring to ASM may want to set this to the ASM stripe size.

Problem can affect any release prior to 11.2.
Please note:
Setting _BACKUP_KSFQ_BUFSZ=1048576 will benefit restore performance overall where you can see that a buffer size of 128Kb is being used. However, if restore validate is also taking a long time then setting this hidden parameter alone may not be enough, particularly if you have hardware multiplexed the original backup (allocated > 1 channel per physical tape device). If this is the case then refer to Note 740911.1: RMAN Restore Performance.